Chromium-Base Alloy and a Production Process Therefor

ABSTRACT

A chromium-base alloy characterized by containing titanium within a range from 0.002 t0 1.5 mass %, with the balance comprising chromium and inevitable impurities, and being produced by melting or casting only. A chromium-base alloy that can be obtained by melting only, has room temperature ductility and a good workability, and has excellent high temperature strength and oxidation resistance even at a high temperature of about 1300° C., as well as a production process therefor are provided.

TECHNICAL FIELD

The invention of the application concerns a chromium-base alloy and a production process therefore. More specifically, the invention of the application concerns a chromium-base alloy obtained only by melting, having room temperature ductility and a good workability, and having excellent high temperature strength and oxidation resistance even at a high temperature of about 1300° C.

BACKGROUND ART

It is said that the usable temperature of nickel-base heat resistance alloys used for rotors and stators in gas turbines is limited to 1100° C. by their melting points, even when countermeasures by coating and air cooling are used in combination. In view of the above, chromium which has a high melting point (1863° C.) as well as a good oxidation resistance and a relatively low density or a chromium-base alloy has been desired as an alternative alloy for nickel-base heat resistant alloys.

However, since chromium and chromium-base alloy have the drawback of lacking in ductility and toughness at room temperature and being brittle at room temperature after nitrogen absorption at a high temperature, chromium-base alloys have not yet been put to practical use as heat resistant structural alloys.

To meet the demand described above, it has been disclosed that an alloy comprising one or more of the metals Al, Ti, Zr, Hf, and Y in an amount from 0.01 to 10 at %, with the substantial balance being chromium, is rolled at a high temperature of 200 to 700° C. after melting and then put to hot rolling (Patent document 1)

Patent Document 1: Japanese Patent Application No. Hei 1-129946

DISCLOSURE OF THE INVENTION

However, the Patent Document 1 describes that the ductile-brittle transition temperature of the obtained chromium alloy is from 200 to 700° C., and this indicates that in the case of adding Al, Ti, Zr, Hf, Y or the like to chromium, the tensile ductility at a temperature lower than the transition temperature thereof, for example, at room temperature cannot be improved. That is, the Patent Document 1 only shows that rolled products of the alloy can be produced under specific rolling conditions and also shows that the ductility can be developed in the chromium alloy only under such specified rolling conditions. Further, since it is essential that the alloys comprising chromium need adjustment of alloy structure by 70% rolling reduction or higher, they involve a drawback that they cannot be utilized, for example, as cast products such as rotors and stators blades of gas turbines.

Further, it is necessary to use a material of high purity as the starting material, reducing the amount of impurities of four kinds of C+N+O+S to be much lower than in usual alloy materials, to 170 mass ppm or less, requiring an increased cost to make the products expensive. Furthermore, this material is not intended at all to have high temperature properties, particularly mechanical properties at about 1300° C. which have been required in recent years, and they are not utilized at all as heat resistant structural alloys.

Then, the invention of the application has been accomplished in view of the foregoing situations and it has a subject of overcoming the problem in the prior art, providing a chromium-base alloy having room temperature ductility and a good workability and also having excellent high temperature strength and oxidation resistance even at a high temperature of about 1300° C., as well as a production process therefore.

Means for Solving the Problems

For solving the subject described above, the invention of the application provides a chromium-base alloy characterized by containing titanium within a range from 0.002 to 1.5 mass % with the balance comprising chromium and inevitable impurities, which can be obtained merely by melting or casting.

Further, the invention of the application provides a chromium-base alloy containing titanium within a range from 0.1 to 0.5 mass %, specifying further the invention described above, and still further, also provides a chromium-base alloy containing from 0.5 to 18 mass % of rhenium, and also a chromium-base alloy containing 30 mass % or less in total of at least one element selected from the group consisting of molybdenum, tungsten, iron, ruthenium, cobalt, rhodium, nickel, platinum, and iridium in addition to the chromium-base alloy containing titanium as described above.

In addition, the invention of the application provides a process for producing a chromium-base alloy described above characterized by only melting or casting the starting material.

EFFECT OF THE INVENTION

According to the invention of the application, a light-weight chromium-base alloy that can be produced only by melting and having a room temperature ductility and a good workability is provided. The chromium-base alloy can exhibit excellent oxidation resistance and high temperature strength even when the total amount of impurity elements about 1000 mass ppm. Further, these properties can be maintained during repetitive use at a high temperature up to about 1300° C. Further, since the chromium-base alloy of the invention of the application can be cast to form products, it can be utilized for rotors and stators blades in gas turbines, as well as various types of heat resistant components such as blades, intake and exhaust valves, and rocker arms of aircraft jet engines and industrial gas turbines and, further, connecting rods and heat resistant wheels of motorcycle and automobile engines. Particularly, a high chromium-base alloy containing 70 mass % or more of chromium can be used, for example, in stator blades of gas turbines, blades of compressors and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a relation between the tensile strain and the amount of Ti at room temperature (25° C.) of a chromium-base alloy of the invention.

FIG. 2 is a view illustrating a relation between the 0.2% compressive yield strength and the temperature of a chromium-base alloy of the invention.

FIG. 3 is a view illustrating the oxidation resistance at 1100° C. of a chromium-base alloy of the invention.

FIG. 4 is a view illustrating the oxidation resistance at 1300° C. of a chromium-base alloy of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention of the application has the features as described above and embodiment thereof are to be described below.

The inventors of the application have found that the low temperature brittleness of chromium is improved remarkably and the room temperature ductility is developed by merely adding only a small amount of titanium to pure chromium, and have accomplished the invention of the application. That is, the chromium-base alloy of the invention of the application is characterized by containing, as a chemical composition, from 0.002 mass % to 1.5 mass % of titanium with the balance comprising chromium and inevitable impurities.

Development of the room temperature ductility of chromium by the addition of titanium described above can be obtained upon adding titanium by about 0.002 mass % to 1.5 mass % and, particularly, a good room temperature ductility of about 15% can be obtained by setting the amount of titanium to about 0.5 mass %. Further, the high temperature strength of the chromium-base alloy is also enhanced by the addition of titanium. For stably obtaining such excellent properties, for example the amount of titanium 0.005 mass % or more.

On the other hand, in the invention of the application, remarkable effect per unit amount of titanium in the chromium-base alloy in providing ductility can be observed in a case of 0.01 mass % or less and, more specifically, 0.009 mass % or less of titanium. As described above, in the chromium-base alloy of the application, sufficient room temperature ductility is provided by the addition of titanium in an extremely small amount.

Further, the inevitable impurities are allowed to be a usual amount, for example, on the ppm order. More specifically, for example, inevitable impurities are up to about 1000 ppm. As the Inevitable impurities, interstitial elements such as oxygen, nitrogen, carbon, sulfur and phosphor are considered to be typical.

Further, the chromium-base alloy provided by the invention of the application is characterized by containing 0.5 to 18 mass. % of rhenium in addition to titanium described above. By the composite incorporation of from 0.5 to 18 mass % of rhenium together with titanium, the chromium-base alloy of the invention of the application is enhanced in tensile ductility at room temperature, as well as in tensile strength and oxidation resistance. For example, a Cr-10Rh-0.1Ti alloy with 0.1 mass % of titanium and 10 mass % of rhenium added to the chromium shows a yield strength at 1300° C. of 150 MPa or more in the state it was in just after melt processing. This is an excellent value, about three times that of pure chromium.

Further, it way be considered for the chromium-base alloy of the invention of the application to contain at least one element selected from the group consisting of molybdenum, tungsten, iron, ruthenium, cobalt, rhodium, nickel, platinum, and iridium, the total amount being 30 mass % or less. These are elements having solid solution hardening effect selected from V, VI, VII, and VIII groups in the periodical table, possible alternative alloy elements. The amount contained is 30 mass % or less in total so as to provide a desired property for the chromium-base alloy. By the addition of these elements, the tensile strength and the tensile ductility are further improved. Preferably, it is generally considered that the total amount is within a range from 0.1 to 10 mass %. In order to ensure the room temperature ductility, it is preferred to incorporate them so as not to exceed 10 mass % per element.

Further, since amounts of inevitable impurities on the ppm order are allowed in the chromium-base alloy of the invention of the application, both in starting materials and in various kinds of additive elements, a purity of a commercial level can be used and there is no requirement for providing high purity.

The chromium-base alloy of the invention of the application described above can be produced most conveniently, by only melting or casting the starting materials which are then blended so as to provide the composition as described above. Alternatively, the Alloy can also be produced by an appropriate single crystal growing method or other known alloy production methods providing that only impurities are included at such a level that do not have undesired effects on the mechanical properties. Of course, further high temperature working such as casting and rolling can be applied to the chromium-base alloy obtained as described above, thereby obtaining desired products.

Examples are to be shown with reference to the appended drawings and preferred embodiments of the invention of the application are to be described more specifically. It will be apparent that the invention is not restricted to the following examples but various forms are possible for the details.

EXAMPLE

Cr-base alloys were produced only by melting, using Cr of the composition shown in Table 1 as a starting base material and adding Ti, Re and Ir as shown in Table 2. Melting was conducted in an argon gas atmosphere by a furnace having a water-cooled copper crucible. Test specimens were cut out from the obtained alloy ingots to form specimens 1 to 13, and various kinds of evaluation tests were conducted. The results are shown in FIGS. 1 to 3.

TABLE 1 Chemical composition (mass %) Cr Ti Re Ir Al O N H S C Fe Si Cu Pd 99.87 0 0 0 0.003 0.018 4006 0.003 0.0002 0.002 0.09 0.006 0.0003 0.0001

TABLE 2 Chemical composition (mass %) Specimen Cr Ti Re Ir Remarks 1 99.87 0 0 0 Comparative Example 2 99.85 0.02 0 0 Example 3 99.82 0.05 0 0 4 99.77 0.1 0 0 5 99.37 0.5 0 0 6 99.87 1.0 0 0 7 99.37 1.5 0 0 8 98.87 0 10 0 Comparative Example 9 98.37 0 18 0 10 89.77 0.1 10 0 Example 11 81.77 0.1 18 0 12 98.87 0 0 1.0 Comparative Example 13 98.77 0.1 0 1.0 Example

FIG. 1 is a view showing a relation between the tensile strain and the Ti content at room temperature (25° C.) for the obtained specimens 1 to 7. While the amount of strain of a single chromium element of the specimen 1 was substantially 0, the amount of strain increased remarkably by the addition of titanium and it was shown that an excellent room temperature ductility, for example 14% strain for the specimen 5 Cr-0.5Ti alloy, could be obtained in the state just after being melt processed.

FIG. 2 is a view showing a relation between the 0.2% compressive yield strength and the temperature for the obtained specimens 1, 4 and 10. For the specimens 4 and 10, the strength was remarkably improved compared with the single chromium element specimen 1 thanks to the solid solution strengthening by the addition of titanium and rhenium. Particularly, for the specimen 10 with composite addition of titanium and rhenium, it was confirmed that the specimen showed a strength about twice as high as the specimen 1 for over the whole range from room temperature to 1300° C. and, particularly, about three times as high at temperature of 1300° C.

FIG. 3 and FIG. 4 are views showing the oxidation resistance at 1100° C. and 1300° C., respectively, of the obtained specimens 1, 10 and 11. The oxidation resistance was evaluated base on the change of weight per unit area of the specimen exposed in the high temperature atmosphere for 200 hours. As apparent from FIG. 3 and FIG. 4, the change of weight of specimens 10 and 11 was about 0, more stable than the single chromium element specimen 1 at a high temperature of 1100° C. or higher, and it was confirmed that specimens 10 and 11 had extremely excellent oxidation resistance for a long time.

Table 3 shows the result of a test for the tensile properties at room temperature of the specimens 1, 8, 10, 12, and 13.

TABLE 3 Mechanical property (room temperature) 0.2% yield Strength Tensile strength at break elongation Specimen (MPa) (MPa) (%) Remarks 1 Cr 167 167 0 Comparative Example 8 Cr—10Re 302 374 4.5 Comparative Example 10 Cr—0.1Ti—10Re 308 383 6 Example 12 Cr—1Ir 185 185 0 Comparative Example 13 Cr—0.1Ti—1Ir 196 217 3 Example

In each of the specimens 8, 10, 12 and 13, the tensile properties were better than the single chromium element specimen 1, and it was shown that both the tensile ductility and the tensile strength were improved further in the specimens 10 and 13 which had composite additions to the titanium than the specimens 8 and 12 which had addition of Re or Ir alone. 

1-2. (canceled)
 3. A chromium-based alloy containing titanium within a range from 0.002 to 1.5 mass % and rhenium within a range from 0.5 to 18 mass %, with the balance comprising chromium and inevitable impurities, and produced by melting or casting only.
 4. A chromium-based alloy according to claim 3 characterized by containing titanium within a range from 0.1 to 0.5 mass %.
 5. A chromium-based alloy containing titanium within a range from 0.002 to 1.5 mass %, and at least one element selected from the group consisting of molybdenum, tungsten, iron, ruthenium, rhodium, nickel, platinum, and iridium in the amount of 30 mass % or less in total, with the balance comprising chromium and inevitable impurities, and obtained by melting or casting only.
 6. A chromium-based alloy according to claim 5 characterized by containing titanium within a range from 0.1 to 0.5 mass %.
 7. A chromium-based alloy according to claim 5 characterized in that the total amount of the one or more elements selected is within a range from 0.1 to 10 mass %.
 8. A chromium-based alloy containing titanium within a range from 0.002 to 1.5 mass % and rhenium within a range from 0.5 to 18 mass %, and at least one element selected from the group consisting of molybdenum, tungsten, iron, ruthenium, cobalt, rhodium, nickel, platinum, and iridium in the amount of 30 mass % or less in total, with the balance comprising chromium and inevitable impurities, and obtained by melting or casting only.
 9. A chromium-based alloy according to claim 8 characterized by containing titanium within a range from 0.1 to 0.5 mass %.
 10. A chromium-based alloy according to claim 8 characterized in that the total amount of the one or more elements selected is within a range from 0.1 to 10 mass %.
 11. (canceled)
 12. A chromium-based alloy according to claim 6 characterized in that the total amount of the one or more elements selected is within a range from 0.1 to 10 mass %.
 13. A chromium-based alloy according to claim 9 characterized in that the total amount of the one or more elements selected is within a range from 0.1 to 10 mass %. 